Various composite piston designs have been proposed for use in internal combustion engines, particularly diesel engines, for a variety of reasons. Among these reasons are to allow higher operating temperatures, increase efficiency and minimize pollutants. Another important reason for the development of a durable composite piston, not discussed in the prior art, is for application in military vehicles. In military service internal combustion engines may be subjected to severe operating conditions. It is necessary to maximize the time an engine may operate with complete coolant loss in order to increase the survivability of the vehicle.
In order to achieve these objectives, prior art composite pistons have relied upon cumbersome flexible mechanical attachment of the piston cap to the metal body, or have provided strain isolation pads between the nonmetallic piston cap and the metal body. These attachment complexities have been required to compensate for the dissimilar coefficients of thermal expansion of the metallic piston body and various piston cap materials.
The former method of attachment usually employs some type of spring device to accommodate thermal expansion and contraction between the fastener and parts fastened. The springs are subject to fatigue wear which increases at high temperatures. When the springs become fatigued they fail to tightly retain the piston cap leading to vibration, noise, dynamic loading and subsequent failure.
The use of strain isolation pads to alleviate thermal expansion differences has inherent problems similar to those associated with the previous method. A typical configuration is made by casting the metallic piston body around the piston cap, which is shaped such that it is retained by the metal around it. Strain isolation pads are placed in the areas where the cap and body interface in order to absorb expansion and prevent thermal stress. In such a design the strain isolation pads are subject to fatigue and crushing, and loss of their resiliency, allows the piston cap to loosen and vibrate.
Also, a number of existing composite piston designs have a metal bolt through the cap, or have a portion of the metal body which extends to the top surface of the cap to provide a means for retaining the piston cap. These designs create a heat short to the metal piston body and significantly limit the maximum operating temperature of the piston.
In addition to extra parts for the purpose of retaining the piston cap, composite pistons have traditionally required piston rings to seal the gap between the piston and cylinder bore. The requirement of piston rings significantly increases the cost of piston manufacture while also increasing the weight of the piston, hence reducing the efficiency.
It is therefore an object of the present invention to provide a composite piston structure with the piston cap attached to the piston body in such a manner as to alleviate thermal stresses.
It is a further object of the present invention to alleviate thermal stress, which would otherwise develop due to different expansion rates for the piston cap and body, without the use of complicated or unreliable mechanical fasteners or strain isolation pads.
A further object of the present invention is to significantly increase the maximum operating temperature of an engine.
A further object of the present invention is to increase the thermal efficiency of an engine.
Another object of the present invention is to decrease the level of pollutants in engine exhaust.
Another object of the present invention is to eliminate the necessity of piston rings in an internal combustion engine utilizing composite pistons.